Precision structural systems carrying a load, such as a telescopic system, are susceptible to disturbances that can produce structural vibrations. Such vibrations may be contributed to the system by components or assemblies of the precision structural systems itself, for example, reaction wheel assemblies of the telescopic system. Since such precision structures tend to have little inherent damping, these vibrations can lead to serious performance degradation. Therefore, an efficient means of damping and isolating the load carried by the precision structures in a controlled manner is of considerable importance.
Both active and passive damping and isolation techniques have been utilized. However, active systems suffer from high cost, low reliability and poor low level threshold performance. On the other hand, passive systems require no power, are often less expensive than active devices and do not drive the structure unstable. Thus, passive damping systems have proven to play a significant role in the overall design of precision structural systems. One such manner of implementing a passive system includes the use of viscous damping and isolation. Viscous dampers and isolators include a fluid reservoir sealed in a damping structure which utilizes viscous fluid shear forces to provide damping and isolation.
One type of viscous damper and isolator, a conventional two parameter system 10, is shown in the mechanical schematic of FIG. 1A. The system includes a damper with a damping function C.sub.A which could be implemented with a piston moving through a liquid in a cylinder. As the piston moves, the damping function is controlled by the size of an orifice allowing liquid to pass from one side of the piston to the other side of the piston. The orifice may be the clearance between the piston and the cylinder. The schematic two parameter system 10 also includes a spring K.sub.A, representing a spring constant. This spring K.sub.A is parallel with the damper C.sub.A and a mass M is attached and applied thereto.
An implemented two parameter system is shown and described in U.S. Pat. No. 4,760,996 to Davis, entitled "Damper and Isolator" and in U.S. Pat. No. 3,980,358 to Davis, issued Sep. 14, 1976 and entitled "Axial Vibration Damper for Floating Bearings". An additional two-parameter system is described in the paper entitled, "A MultiAxis Isolation System for the French Earth Observation Satellite's Magnetic Bearing Reaction Wheel," by D. Cunningham, P. Davis and F. Schmitt, Proceedings of the ADPA/AIAA/ASME/SPIE Conference on Active Materials and Adaptive Structures, Nov. 5-7, 1991. The paper describes a six degree of freedom isolation system using viscous dampers.
A mechanical schematic of an additional damper and isolator is shown by the three parameter system 12 of FIG. 1B. The three parameter system 12, includes a spring K.sub.B, representative of a spring constant, in series with a damper C.sub.A representative of a damping function. The three parameter system further includes a spring K.sub.A representative of a spring constant, parallel to the damper C.sub.A and spring K.sub.B. A mass M or load is applied to the system. Such a mechanical schematic is discussed in the textbook entitled "Mechanical Vibrations Theory and Applications, F. S. Tse, I. E. Morse, R. T. Hinkle, Second Edition 1978, pages 106-107.
It can be said that a two parameter system is simply a special case of a more general three parameter system; that is, when K.sub.B is equal to infinity. Most practical isolation systems have a finite value of K.sub.B due to some compressibility of the fluid or volumetric compliance in the housing of a particular viscous isolator. Nevertheless, they essentially act as a two parameter system because of the relatively large value of K.sub.B compared to K.sub.A ; K.sub.A being the stiffness in parallel with the damper C.sub.A.
One of the prime concerns in the field of vibration isolation is the proper use of isolators to isolate loads from external vibrations under various load configurations with respect to a desired natural frequency. The natural frequency is the frequency at which a freely vibrating mass system oscillates once it has been deflected. In the case where external vibrations occur over a wide frequency range, resonance is said to exist when the natural frequency of the viscous isolator coincides with the frequency of the external vibration or excitation forces. Resonance causes magnification of the external vibration and may be harmful to the isolated structure if not controlled within reasonable limits.
Two parameter systems lack adequate performance when isolation at frequencies substantially above resonance is important, and/or when reduced amplification at resonance is important as is shown by the transmissibility plot 200 of FIG. 1C. The transmissibility plot 200 shows two dashed curves 204 and 202 representing two parameter systems of transmissibility versus frequency ratio, i.e., frequency of external vibrations to a natural frequency. Dashed line 202 represents a 50% damped system and shows that isolation at high frequencies is extremely poor. Dashed line 204 represents a 0% damped system and although isolation is much better at high frequencies, amplification at resonance is unacceptable. The three solid curves 206, 208 and 210 represent transmissibility versus frequency ratio for a three parameter system. As is shown, with a 100% damped system at resonance, the amplification at resonance is approximately a factor of 4. The same as for a two parameter 50% damped system. As shown by solid line curves 206 and 208, although isolation at high frequency is improved with a lesser damped three parameter system, i.e., curve 206 a 20% damped system and curve 208 a 50% damped system, amplification at resonance also increases. This indicates that a fundamental rule generally exists when optimizing a three parameter system; the rule being that the frequency at which maximum damping occurs should be equal to the resonance frequency. However, other basis of optimization may be more appropriate depending on the application. As is shown in FIG. 1C, a three parameter system has the primary advantage over the two parameter system in that three parameter system provides better isolation at high frequencies with equal or less amplification at the resonance frequency.
Although prior modeling of two and three parameter systems indicates that a three parameter system provides better isolation at high frequencies with equal or less amplification at the resonance frequency, an efficient three parameter viscous isolator providing such isolation is not known. Therefore, a need for such a three parameter viscous isolation system exists.